The present invention relates to a new and improved method for checking coins and to a coin checking apparatus for the performance of the aforesaid method.
A heretofore known method of checking coins contemplates conducting the coins to be checked in succession through both alternating-current fields of two successively arranged oscillator coils dimensioned such that at the first oscillator coil, when influenced by a "proper" coin, the oscillations of the oscillator just begin to breakdown and at the second oscillator coil the oscillations do not yet breakdown. The different dimensions of both oscillator coils govern the permissible stray range for a certain type of coin. Only when, as described, the one oscillator stops and the second does not, is the coin accepted. While this technique can be put into practice economically, still it is qualitatively unusable. The criterion "stopping of the oscillations of the oscillator", of all conceivable criteria, is the one which is most temperature-dependent, and even with constant environmental conditions is not accurately reproducable.
According to a further prior art process for checking coins there is employed a bridge-measurement circuit. This technique has become known in numerous modifications. While it has the advantage of extreme accuracy, it is associated with the drawback that it is difficult to fabricate at relatively high manufacturing costs and equally is not suitable for use as a multiple-coin checking device. It is characterized by a bridge which in one of its branches is pre-loaded either by an original coin or an appropriate electrical magnitude and the other branch is loaded by the coin to be checked. What is evaluated is the one-time self-adjustment of the null voltage, which only can be obtained if the strictly predetermined original coin is compared with an equivalent coin.
Further state-of-the-art methods utilize the damping of a transformer by means of a coin moving therepast which is to be checked, and owing to the influence of the coin there is reduced the HF--no load--amplitude at the secondary. The degree of damping, i.e. the maximum amplitude of a negative measurement voltage at the secondary, is used as the criterion for the recognition of a coin type.
Another group of already known coin checking methods employ the evaluation of positive measurement voltage amplitudes at the secondary, as such occur during de-tuning of symmetrically constructed differential-transformer probes.
Both the evaluation of the maximum damping at the secondary as well as the evaluation of the maximum non-symmetry at the secondary result in good recognition of the different coin types. But both techniques are associated with the drawback that they require complicated circuit design to obtain good temperature stability. In any event both of the last-mentioned methods enable, by means of a single measurement arrangement, the recognition of different types of coins, since there only must be utilized a corresponding number of window circuits in order to monitor the measurment voltage regions corresponding to the individual coin types to be checked. However, with both methods the circuits required are critical and expensive to manufacture.